The ultimate goal of this proposal is to determine the cause(s) of impaired glucose tolerance leading to type 2 diabetes. An improved understanding of the underlying pathophysiology of the early events should provide a rational basis for the development of improved methods of early diagnosis and ultimately treatments for type 2 diabetes. The pancreatic beta cell response to nutrient oversupply and obesity associated insulin resistance is compensatory insulin hypersecretion in order to maintain euglycemia. Diabetes, and early states of glucose intolerance only develop in those who develop beta cell dysfunction. Although beta cell mass has been found to decline by ~50% in those with prediabetic states of glucose intolerance, controversy exists as to whether defects in the beta cell secretory response is due to deficient synthetic machinery or functional defects in glucose sensing and insulin secretion. This proposal will test the hypothesis that intrinsic or acquired defects in insulin biosynthesis characterize states of glucose intolerance, that these defects result in depletion of a rapidly mobilizable pool of insulin necessary for fully efficient beta cell function in response to nutrient stimuli. Insulin biosynthetic rates and relative sizes of intracellular beta cell pools will be measured for the first time in vivo in streptozotocin treated rats, and in subjects with normal, impaired glucose tolerance and diabetes by the novel method of mass isotopomer distribution analysis.